1. Field of the Invention
The present invention relates to a transmission circuit for use in a communication apparatus for mobile telephony, wireless LAN, or the like. More particularly, the present invention relates to a small-size transmission circuit which outputs a transmission signal having high linearity independently of a magnitude of an output power and operates with high efficiency, and a communication apparatus employing the transmission circuit.
2. Description of the Background Art
There is a demand for a communication apparatus for mobile telephony, wireless LAN, or the like which can secure the linearity of a transmission signal and operate with low power consumption no matter whether it operates within a large or small power. In such a communication apparatus, a small-size transmission circuit is employed which outputs a transmission signal having high linearity independently of a magnitude of the output power and operates with high efficiency. Hereinafter, conventional transmission circuits will be described.
As a conventional transmission circuit, for example, there is a transmission circuit which utilizes a modulation method, such as quadrature modulation or the like, to generate a transmission signal (hereinafter referred to as a quadrature modulation circuit). Note that the quadrature modulation circuit is widely known and will not be described. As a conventional transmission circuit which outputs a transmission signal having high linearity with higher efficiency than that of the quadrature modulation circuit, for example, there is a transmission circuit 500 illustrated in FIG. 20. FIG. 20 is a block diagram illustrating a configuration of the conventional transmission circuit 500. In FIG. 20, the conventional transmission circuit 500 comprises a signal generating section 501, an angle modulating section 502, a regulator 503, an amplitude modulation section 504, and an output terminal 505.
In the conventional transmission circuit 500, the signal generating section 501 generates an amplitude signal and a phase signal. The amplitude signal is input to the regulator 503. The regulator 503 supplies a voltage depending on the input amplitude signal to the amplitude modulation section 504. The phase signal is input to the angle modulating section 502. The angle modulating section 502 subjects the input phase signal to angle modulation to output an angle-modulated signal. The angle-modulated signal output from the angle modulating section 502 is input to the amplitude modulation section 504. The amplitude modulation section 504 subjects the angle-modulated signal to amplitude modulation using the voltage supplied from the regulator 503, to output an angle-modulated and amplitude-modulated signal. This modulated signal is output as a transmission signal from the output terminal 505. Note that such a transmission circuit 500 is called a polar modulation circuit.
As a conventional transmission circuit which outputs a transmission signal having high linearity with higher efficiency than that of quadrature modulation circuits, for example, there is a transmission circuit 600 illustrated in FIG. 21 which is called LINC (Linear Amplification using Nonlinear Components). FIG. 21 is a block diagram illustrating a configuration of the conventional transmission circuit 600. In FIG. 21, the conventional transmission circuit 600 comprises a constant-amplitude wave generating circuit 601, an amplifier 602, an amplifier 603, and a combining circuit 604.
The constant-amplitude wave generating circuit 601 outputs two modulated signals having different phases and a constant amplitude (hereinafter referred to as a constant-amplitude signal) based on an input signal. The two constant-amplitude signals output from the constant-amplitude wave generating circuit 601 are amplified in the amplifier 602 and the amplifier 603, and are then input to the combining circuit 604. The combining circuit 604 combines an output signal S1 of the amplifier 602 with an output signal S2 of the amplifier 603, and outputs the combined signal as a transmission signal S0.
Here, the transmission signal S0, the output signal S1 of the amplifier 602, and the output signal S2 of the amplifier 603 can be represented by:S0(t)=m(t)exp[jθ(t)]=S1(t)+S2(t)  (10)S1(t)=Mx exp [j{θ(t)+φ(t)}]  (11)S2(t)=Mx exp [j{θ(t)−φ(t)}]  (12)
                              φ          ⁡                      (            t            )                          =                              cos                          -              1                                ⁡                      [                                          m                ⁡                                  (                  t                  )                                                            2                ⁢                M                ⁢                                                                  ⁢                x                                      ]                                              (        13        )            where m(t) represents an amplitude component of the transmission signal S0, θ(t) represents a phase component of the transmission signal S0, Mx represents a magnitude of an amplitude of the output signal S1 of the amplifier 602 or the output signal S2 of the amplifier 603, and ψ(t) represents a deviation of a phase of the output signal S1 or the output signal S2 from the transmission signal S0.
FIG. 22 is a diagram for specifically describing an operation of the conventional transmission circuit 600. Referring to FIG. 22, the conventional transmission circuit 600 outputs the transmission signal S0 which has been increased by reducing the phase deviations of the output signal S1 and the output signal S2 from the transmission signal S0 (see (a) of FIG. 22). Also, the transmission circuit 600 outputs the transmission signal S0 which has been decreased by increasing the phase deviations of the output signal S1 and the output signal S2 from the transmission signal S0 (see (b) of FIG. 22). In other words, the transmission circuit 600 can control a magnitude of the transmission signal S0 by controlling the phase deviations of the two constant-amplitude signals output by the constant-amplitude wave generating circuit 601.
However, in the conventional transmission circuit 600, since the transmission signal S0 is generated by combining the output signal S1 and the output signal S2, it is difficult to achieve the desired transmission signal S0 if the output signal S1 and the output signal S2 include a phase error or an amplitude error.
Therefore, a conventional transmission circuit called LINC has been disclosed in which a phase error and an amplitude error included in the output signal S1 and the output signal S2 are corrected (see, for example, Japanese Laid-Open Patent Publication No. 5-37263). Hereinafter, Japanese Laid-Open Patent Publication No. 5-37263 is referred to as Patent Document 1. FIG. 23 is a block diagram illustrating a configuration of a conventional transmission circuit 700 disclosed in Patent Document 1. In FIG. 23, the conventional transmission circuit 700 comprises a constant-amplitude wave generating circuit 601, an amplifier 602, an amplifier 603, a combining circuit 604, a phase detector 701, a variable phase shifter 702, an amplitude difference detector 703, and a variable attenuator 704.
In the conventional transmission circuit 700, the phase detector 701 detects a phase error included in an output signal S1 of the amplifier 602. Based on the detected phase error, the variable phase shifter 702 corrects a phase of a constant-amplitude signal generated by the constant-amplitude generating circuit 601. The amplitude difference detector 703 detects an amplitude error included in an output signal S2 of the amplifier 602. Based on the detected amplitude error, the variable attenuator 704 corrects an amplitude of the constant-amplitude signal generated by the constant-amplitude generating circuit 601. Thereby, the conventional transmission circuit 700 can achieve a desired transmission signal S0.
However, the conventional transmission circuit 500 (see FIG. 20) cannot output a transmission signal having an output power smaller than a predetermined output power (i.e., there is a lower limit of the output power of the transmission signal). FIG. 24 is a diagram illustrating exemplary output characteristics of the conventional transmission circuit 500. In FIG. 23, the horizontal axis represents an amplitude signal output from the signal generating section 501, and the vertical axis represents the output power of the transmission signal. As illustrated in FIG. 24, in the conventional transmission circuit 500, it is difficult to cause the amplitude modulation section 504 to perform a linear operation, so that a transmission signal having high linearity cannot be output, in a region in which the output power is small (i.e., a region in which the amplitude signal is small).
In the conventional transmission circuit 600 (see FIG. 21), as described above, since the output signal Si and the output signal S2 having different phases are combined to generate the transmission signal S0, it is difficult to achieve the desired transmission signal S0 due to a phase error or an amplitude error included in the transmission signal SI and the transmission signal S2. Also, in the transmission circuit 600, since the output signal S1 and the output signal S2 having different phases are combined to generate the transmission signal S0, a high-efficiency operation cannot be necessarily achieved, depending on the magnitude of the output power.
In the conventional transmission circuit 700 (see FIG. 23), a number of parts (e.g., the phase detector 701, the variable phase shifter 702, the amplitude difference detector 703, and the variable attenuator 704) are required to correct a phase error or an amplitude error included in the output signal S1 and the output signal S2. Therefore, the conventional transmission circuit 700 unavoidably has a large circuit scale. Also, in the conventional transmission circuit 700, since the output of each of the amplifier 602 and the amplifier 603 is branched, the branching causes a loss, so that the power consumption of the transmission circuit increases.